The consumer-electronics site reported on the results from a plug-in program pioneered by Internet juggernaut Google. The Google.org project, dubbed RechargeIT, recently took four Toyota Priuses and two Ford Escapes and converted them to plug-in hybrid vehicles that could be rejuiced from a standard wall socket. Google.org employees then used them as fleet vehicles.

What the Google group found was that the plug-in hybrid vehicles -- costly conversions of existing hybrids, not any of the projected plug-ins like the Chevy Volt or Toyota Prius in the works from automakers -- only saved about 88 gallons of fuel a year. The annual savings translated into about $150 to $250 less in fuel bills, with added electricity costs factored in. At $15,000 for the plug-in add-on, the conversion of existing cars would take nearly a century to pay for itself with gas at $3 a gallon.

If gas surges to $5 a gallon, the payoff is shorter -- just 30 years.

Still, the RechargeIT project says the plug-ins emit almost 5000 pounds less of carbon dioxide over a 12,000-mile year. That figure is about 50 percent better than a standard Prius. The gas mileage for the Prius hit 66.2 mpg in testing, a good measure of improvement over the stock Prius' 44.6 miles per gallon. Both of those figures were achieved in city driving; highway efficiency for plug-in hybrid cars might be even higher, RechargeIT told C/Net.